It is desirable to produce products, including liquid fuels such as ethanol, from renewable resources, e.g., biomass, because of the limited supply of easily recovered petroleum and natural gas and the increasing price of recovery. In addition, the use of products produced from biomass will reduce the buildup of carbon dioxide in the atmosphere.
It is particularly desirable to produce products from biomass in such a manner that a maximal amount of the energy content, carbon content and mass content contained within such materials is transferred to such products. Current processes that use biomass, however, are not efficient in such transfer.
Traditional renewable-based chemicals, such as ethanol and lactic acid, have been produced from agricultural grains. For example, sugars can easily be produced from the starches of corn, wheat or other grains by enzymes or acid hydrolysis or be recovered from sugar-containing crops such as sugar cane at high yield. Such sugars can then be easily converted to ethanol by, for example, natural yeast organism fermentation. Although for yeast fermentation about 96% of the energy in the sugar material is transferred into ethanol, only about 67% of the carbon in the sugar material is transferred into ethanol. This low carbon efficiency is due to yeast producing two moles of carbon dioxide for each two moles of ethanol produced from one mole of glucose. This process results in a maximum mass (or dry weight) yield of about 52%.
However, the conversion of biomass into ethanol is not as efficient as the conversion of sugars into ethanol. Biomass is a complex material, containing not only starch and other sugars but also structural parts (e.g., stems, leaves, cobs, etc.) that are complex and contain several components, including cellulose, hemicellulose, and lignin. For example, about 45% of a corn plant is the grain (corn kernel) and the structural parts make up the remaining about 55%. Each of these components comprises about 70% carbohydrate. As such, the starch fraction of the kernel of corn, one of the most productive grains, is only about 32% (45%×70%) of the entire plant's mass and energy content.
Biomass is a heterogeneous mixture, the components of which are intermingled and cannot be separated by simple physical means. Typically, biomass includes two main fractions: carbohydrates and non-carbohydrates. The carbohydrate fraction includes cellulose, hemicellulose, starch and sugars. Cellulose, hemicellulose and starch typically include sugars such as glucose, xylose, arabinose, mannose, etc. The non-carbohydrate fraction includes lignin, which is a complex phenolic material, as well as proteins, resinous materials and minerals. The carbohydrate fraction of a typical biomass, such as wood, may comprise about 60% to about 70% of the total material on a dry weight basis, while lignin and other non-carbohydrates comprise the remainder. Other forms of biomass, however, may have a quite low proportion of carbohydrates. For example residues from the forestry industry may include bark, which may be quite low in carbohydrate (often less than about 25% of the total material on a dry weight basis).
A corn plant is one of the most efficient examples of biomass used to produce ethanol. However, since a corn-based process converts only the starch fraction of the corn plant into ethanol and since starch is only about 32% of the plant's mass and energy content (as described above), the energy yield from the whole corn plant is only about 31% (i.e., 32%×96%), and the dry weight yield is only about 17% (i.e., 32%×52%). Such yields represent an important limitation on the use of grain crops for conversion to renewable-based fuels and chemicals.
In order to overcome this limitation, and to avoid competition for grain as an important human and animal food source, there have been many efforts to produce renewable chemicals and fuels from complex cellulosic biomass, especially the non-grain parts of crops as well as non-food plants such as woody plants and grasses. The structural (non-edible) parts of a food plant (e.g., the stover from corn plants or the straw from wheat plants) can comprise up to about 70% carbohydrate for corn on a dry weight basis, in the form of cellulose and hemicellulose. If one could efficiently process all of the carbohydrate fractions of the plant structure using a direct yeast-based ethanol fermentation, for example, one could convert up to about 67% (70%×96%) of the energy stored in the plant structure to ethanol. However, only about 36% (70%×52%) of the dry weight could be converted to ethanol.
Woody plants and grasses have high yields of total biomass per unit of land area and thus can provide an attractive economic basis if one can utilize them efficiently. In addition it has been found that even for certain food crops, such as sugar cane and corn, the overall biomass yield can be improved by changing the plant breeding criteria away from traditional targets, such as sucrose concentration and kernel yield, to total energy yield per acre of planted land.
The conversion of cellulosic biomass to ethanol has been a very active field of research because of the availability of structural biomass and potential higher yield of ethanol compared to using only grains. There are, however, a number of unresolved technical problems that need to be solved before this approach can provide a significant source of renewable based fuels and chemicals.
One method to convert biomass to ethanol is a direct analogy to the traditional ethanol fermentation process, based on the yeast fermentation of sugars from starch derived from, for example, corn and wheat. However, the sugars in this case are derived from the hemicellulose and cellulose fractions of biomass.
There has been research on extracting sugars from biomass for many years, and a number of methods have been developed. One method is acid hydrolysis of the carbohydrate fractions. Since the main components of the carbohydrate fraction of biomass are sugar polymers, they can be hydrolyzed using an acid catalyst in water. Many different mineral and organic acids, and a wide range of conditions, have been attempted. One concern with acid hydrolysis is the reaction of the product sugars with acid to produce degradation byproducts that reduce sugar yield and may inhibit fermentation.
An alternative process is the use of enzymes to hydrolyze the carbohydrates. Various hemicellulase and cellulase enzymes have been found in nature and developed as catalysts for the breakdown of biomass. In addition, it is often preferred to pretreat the biomass to separate the lignin and hemicellulose from the cellulose before hydrolysis. Many different combinations of pretreatment and hydrolysis have been developed.
Biomass, in contrast to starch, contains a number of different kinds of sugars. Cellulose is largely a glucose polymer, but hemicellulose is a very complex amorphous and branched polymer that usually contains several different sugars. A major component of hemicellulose contains xylose and arabinose, so-called C5 sugars. Each type of biomass contains a different mix of hemicellulose sugars.
The natural yeasts used for ethanol fermentation only ferment sugars such as glucose, sucrose or other so-called C6 sugars. They typically do not ferment the C5 sugar xylose (found in hemicellulose) to ethanol. Thus, research continues to be conducted to find an efficient way to convert the mixed sugars derived from biomass to ethanol.
Even if these technical issues are resolved, however, there is still the limitation that only part of the biomass (i.e., the carbohydrate fraction) can be converted by direct fermentation to products. And because of the limitation of the direct biological approach, there remains a significant yield loss.
The non-carbohydrate fraction of biomass contains many non-fermentable components such as lignin. Some of these components may also be inhibitory to the ethanol-producing organism. Lignin is often burned to offset energy use in the fermentation and recovery process or sold as excess power or heat energy. For example, many sugar cane mills burn left over plant structural material or bagasse.
An alternative to fermentation of the carbohydrates in biomass to ethanol is the conversion of all of the biomass including the carbohydrates and lignin by thermochemical means, such as gasification, to an intermediate syngas. Syngas is comprised of carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2) as well as other minor components such as tars and sulfur compounds.
There are a variety of gasification processes to produce syngas, each of which typically cracks carbon-containing materials to produce a gaseous mixture containing CO, CO2 and H2, along with tars and particulates from the mineral fraction of the raw material. A second reactant, such as steam, air, or oxygen, can be added to the process. Alternatively, gasification can be conducted in the absence of such reactants; this process is often called pyrolysis. The product syngas usually must be filtered and then adjusted in composition depending upon the use. In addition to filtration, the syngas may be cleaned up, for example to remove tars by various means, such as scrubbing or reaction, and to remove specific impurities, such as sulfur compounds. The composition of the syngas can be adjusted by various chemical means or by separation of certain components. Hydrogen content can be adjusted by the water-gas shift reaction between carbon monoxide and steam to give hydrogen and carbon dioxide. Pressure swing absorption can be used, for example, to separate and purify hydrogen from syngas.
The thermochemical conversion of biomass materials to syngas results in a decrease in energy of the products along with the production of byproduct heat. About 70% of the energy content of biomass is converted into syngas by gasification.
After the conversion of all of the components of biomass to syngas, two options have been proposed for the conversion of the syngas to ethanol or other products. One approach is the conversion of the syngas by a fermentation process. There are a number of organisms that can utilize syngas to produce products, such as ethanol. These fermentations are less able to conserve energy than are sugar fermentations, typically being about 80% efficient in converting the energy in the syngas into product chemical energy. Thus the overall transfer of biomass energy to ethanol by such a combined gasification and fermentation process is about 56% (70%×80%).
A second approach is the use of a catalytic chemical process to convert syngas to products such as ethanol. This process requires a catalyst as well as high temperature and pressure. The catalytic chemical process has the same overall chemistry and thermal efficiency as the gasification plus fermentation process, i.e., 56%, since the overall chemistry is the same.
An advantage of the thermochemical route to produce, for example, ethanol is that it can potentially convert more of the fractions of the plant into products because it can convert the lignin fraction. On the other hand, this route suffers from a theoretical yield loss in each step, so overall its energy efficiency is limited. The thermochemical process wastes the energy stored in the carbohydrate fraction of the biomass by degrading the carbohydrates to syngas, which is less efficient than a biological conversion of the carbohydrates to a desired product.
For each of these described processes, there is also a limit on what product can be produced efficiently. Direct fermentation of biomass to a desired product is limited by the biochemical pathways that can be discovered or engineered into an organism as well as by the need to maintain the viability of the organism and to use available substrates. Thermochemical processing of biomass also has limitations because, for example, fermentation of syngas is also limited by organism constraints, whereas chemical catalysts are limited by their ability to convert syngas selectively to a desired product rather than to a complex mixture.
Thus, there remains a need to produce products from biomass such that energy, carbon and mass contained in all parts of the biomass are efficiently transferred to the products. It would be desirable to have a process that maximizes the use of both the carbohydrate and non-carbohydrate fractions of biomass to produce a product.